The main difference between those technologies is presented in the Forum Nokia Java FAQ:
PersonalJava provides a runtime environment based on the Java 1.1 platform. It is not Java 2 platform like Java 2 Micro Edition (J2ME), which is tailored for consumer and embedded devices, such as mobile phones. In other words, Personal Java is an older and bigger environment than J2ME MIDP but it also provides a richer development platform.